Today there are a wide range of intravascular prostheses on the market for use in the treatment of aneurysms, stenoses, and other vascular irregularities. Balloon expandable and self-expanding stents are well known for restoring patency in a stenosed vessel, e.g., after an angioplasty procedure, and the use of coils and stents are known techniques for treating aneurysms.
Previously-known self-expanding stents generally are retained in a contracted delivery configuration using an outer sheath, then self-expand when the sheath is retracted. Such stents have several drawbacks, for example, the stents may experience large length changes during expansion and may shift within the vessel prior to engaging the vessel wall, resulting in improper placement. Additionally, many self-expanding stents have relatively large delivery profiles because the configuration of their struts limit compression of the stent. Accordingly, such stents may not be suitable for use in smaller vessels, such as cerebral vessels and coronary arteries.
Other drawbacks associated with the use of coils or stents in the treatment of aneurysms is that the coils or stents, when deployed, may have a tendency to remodel or straighten a delicate cerebral vessel, which may cause further adverse consequences. Moreover, such devices may not adequately reduce blood flow from the cerebral vessel into the sac of the aneurysm, which may increase the likelihood of rupture. Generally, if a greater surface area is employed to cover the sac, the delivery profile of the device may be compromised due to the increased surface area, and the device also may be more rigid and cause remodeling of the vessel.
For example, PCT Publication WO 00/62711 to Rivelli describes a stent comprising a helical mesh coil having a plurality of turns and including a lattice having a multiplicity of pores. The lattice is tapered along its length. In operation, the plurality of turns are wound into a reduced diameter helical shape, then constrained within a delivery sheath. The delivery sheath is retracted to expose the distal portion of the stent and anchor the distal end of the stent. As the delivery sheath is further retracted, the subsequent individual turns of the stent unwind to conform to the diameter of the vessel wall.
The stent described in the foregoing publication has several drawbacks. For example, due to friction between the turns and the sheath, the individual turns of the stent may bunch up, or overlap with one another, when the delivery sheath is retracted. In addition, once the sheath is fully retracted, the turns may shift within the vessel prior to engaging the vessel wall, resulting in improper placement of the stent. Moreover, because the distal portion of the stent may provide insufficient engagement with the vessel wall during subsequent retraction of the remainder of the sheath, ambiguity concerning accuracy of the stent placement may arise.
When utilizing stents in interventional procedures, it may be advantageous to deliver therapeutic agents to a vessel wall via the surface of the stent. Such drug eluting stents have many advantages, such as controlled delivery of therapeutic agents over an extended period of time without the need for intervention, and reduced rates of restenosis after angioplasty procedures. Typically, the drug is disposed in the matrix of bioabsorbable polymer coated on an exterior surface of the struts of the stent. The drug gradually elutes from the polymer or is released into a vessel wall as the polymer biodegrades. The quantity of the therapeutic agent provided by the stent generally is limited by the surface area of the struts. Increasing the surface area of the struts may enhance drug delivery capability, but may compromise the overall delivery profile of the stent. Accordingly, there exists a need for a prosthesis having a reduced delivery profile and enhanced drug delivery capabilities.
Helically wound, such as described in U.S. Pat. No. 4,503,569 to Dotter, lack a controllable degree of surface area. For example, the stent is only in contact with a narrow portion of the bodily vessel and offers limited support for the tissue between adjacent turns or winds. Moreover, adjacent turns may move relative to each other, resulting in larger gaps between some turns as compared to others. Still further, radial compressive forces may become concentrated on only a few turns of the stent, rather than being distributed over a longer length of the stent surface.
Other helical stent designs have attempted to overcome this problem by increasing the width of the contact area. For example, the width of the helical body of the stent may be widened so as to resemble a ribbon, such as disclosed in U.S. Pat. No. 5,833,699 to Chuter. Nevertheless, such designs still may not adequately distribute radial compressive forces to adjacent turns of the stent. Also, because the space between adjacent turns may vary according to the inner diameter of the bodily vessel, the extent of any such force distribution may be variable.
In view of these drawbacks of previously known devices, it would be desirable to provide apparatus and methods for an implantable vascular prosthesis that may be configured for use in a wide range of applications including, but not limited to, treating aneurysms, maintaining patency in a vessel, and delivering drugs to a vessel wall.
It also would be desirable to provide apparatus and methods for a vascular prosthesis that is flexible enough to conform to a natural shape of a vessel without substantially remodeling the vessel.
It further would be desirable to provide apparatus and methods for a vascular prosthesis that facilitates controlled deployment of the prosthesis at a desired location within a vessel.
It still further would be desirable to provide apparatus and methods for a vascular prosthesis that has a selectable surface area to facilitate in-vivo delivery of therapeutic agents.
It yet further would be desirable to provide apparatus and methods for a helical vascular prosthesis that reduces the area of unsupported tissue between adjacent turns.
It also would be desirable to provide apparatus and methods for a helical vascular prosthesis that enhances distribution of compressive forces over multiple adjacent turns.